UV-curable dielectric compositions

Abstract

 A printable dielectric composition comprising finely divided particles of talc and/or mica dispersed in a curable liquid composition containing acrylated rubber modified epoxy resin oligomer, acrylated polydiene oligomer and alkyl acrylate.

Description

Field of Invention

[0001] The invention is directed to novel UV-curable dielectric compositions and especially to such compositions for use in membrane touch switches.

Background of the Invention

[0002] The membrane touch switch is a normally open, low-voltage, pressure-sensitive device currently being used in a wide variety of applications, including appliances, electronic games, keyboards, and instrumentation. It is usually fabricated as a three-layer sandwich with the conductive traces printed on the inner sides of the top and bottom layers which are separated by a spazcer sheet. Pressure applied to the top layer establishes momentary electrical contact between the top and bottom layers through punched openings in the spacer sheet. Both flexible and rigid switches are available. The former are typically printed over a flexible polyester base, while the latter use a printed circuit board bottom panel.

[0003] Ease of design and manufacture allow touch switches to cost less than their electromechanical counterparts. Nevertheless, it is still imperative that they be made from high reliability electronic materials, and that these materials be compatible with each other. Since the high cure temperatures of the many inks available for cermet applications are not suitable for polymeric substrates, many polymer thick film conductors and dielectrics have been developed for this application. A variety of chemistries is currently used for both types of inks, and a variety of processing options are used as well.

[0004] In practice, most manufacturers first select a conductive ink, then look for a compatible dielectric. The selection is especially critical in this application, since the dielectric is used both to insulate the conductor, to allow crossovers, and to encapsulate it to prevent environmental damage. However, lack of adequate adhesion of the dielectric to the substrate and/or to the conductive ink has resulted in limited market penetration for many dielectric compositions, especially those which are UV curable.

[0005] Existing manufacturing processes dictate that the dielectric be screen-printable, and either thermally curable or UV light curable. Faster cures which can be obtained with the latter make it the more cost-effective approach, and the wide availability of UV curing units makes this a practical route. The dielectric must be compatible with the conductive ink, and must meet certain performance standards. It must cure to a flexible, abrasion-resistant film, free of pinholes with good adhesion to the substrate and to the conductive ink. Crossover applications also require that the conductive ink have good adhesion to the dielectric, and, frequently, good adhesion of the dielectric to itself is also specified. Electrical requirements call for a low dielectric constant, high insulation resistance, and high breakdown voltage. The physical and electrical properties must not degrade under a variety of environmental conditions.

[0006] In an assembled switch, dielectric failure can lead to either electrical or physical breakdown of the switch. Both materials vendors and switch manufacturers rigorously test components under fresh and accelerated aging conditions to decrease the probability of this occurrence. Electrical failure implies that shorting has occurred because of pinholes, the presence of conductive impurities in the formulation, dielectric failure under load, or other stressful environmental conditions. Physical failures originate from blistering, softening, or cracking, any of which can occur during the manufacturing process or during use. Blistering may be due to incompatibility of the dielectric with the conductor or the substrate, as well as from moisture susceptibility. Softening can occur under high humidity conditions or with solvent from a conductor ink, and cracking can result from the inherent brittleness of a cured composition. All of these problems can be prevented with appropriately formulated inks.

[0007] A more difficult problem is that of adhesive loss, and since this is intimately related to the substrate, the problem is compounded by the large number of available substrates. While polyester films are the most widely used in touch switches, polycarbonate and polyimide films are occasionally encountered. Each film manufacturer typically offers several grades of each product, with different surface characteristics due to variable processing techiques and/or surface pretreatments. The films may also be given a heat treatment to reduce shrinkage in later curing steps.

[0008] Different polyester films have different physical surfaces. For example, both Mylar®EL 500 and 500D⁽⁷⁾ polyester films show evidence of rough surfaces, due to slip pretreatment to allow easy handling of these films, while Melinex®O⁽⁶⁾ polyester film has an extremely smooth surface. The Mylar®500D polyester film has much smaller particulates than the Mylar®EL 500 polyester film, giving it a clear appearance, while the Mylar®EL 500 has a cloudy apperance. The Melinex®O polyester film is also very clear, but suffers from poor handleability and tends to stick to itself. As might be predicted adhesion to these surfaces is quite variable and indirectly related to surface smoothness - the Melinex®O polyester film generally giving the poorest values. Since membrane switch manufacturers often select their substrates not for microscopic structure but for reasons of cost, dimensional stability and visual appearance, the physical surface characteristics are frequently overlooked, yet may be critical to the performance of an ink from the standpoint of adhesion.

Summary of the Invention

[0009] The invention is therefore directed in its primary aspect to an improved screen printable dielectric composition having superior adhesion to a wide variety of substrates which is a printable dielectric composition comprising

(a) 25-35% wt. finely divided particles of an inorganic adhesion agent selected from talc, mica and mixtures thereof dispersed in

(b) 75-65% wt. UV curable liquid composition comprising:

(1) 20-50% wt. acrylated rubber-modified epoxy resin oligomer;

(2) 5-25% wt. acrylated polydiene oligomer; and

(3) 35-75% wt. alkyl acrylate.

[0010] In its secondary aspect the invention is directed to membrane touch switches comprising upper and lower flexible layers having facing electrically conductive areas separated by an adherent spacer layer of the above-described composition.

[0011] In yet another aspect the invention is directed to membrane touch switches comprising upper and lower flexible layers having facing electrically conductive areas separated by an adherent spacer layer and having electrically conductive traces leading therefrom encapsulated within a layer of the above-described composition.

[0012] In a still further aspect, the invention is directed to a membrane touch switch comprising upper and lower flexible layers at least one of which layers has a plurality of overlying electrically conductive areas each separated from the other by a layer of the above-described composition.

Adhesion Testing

[0013] The mose widely accepted criterion for measuring the adhesion of membrane switch materials is the tape test described by ASTM D3359-78, Method B. For films under 5 mils thickness, it requires that a 10 by 10 grid pattern be made with a sharp cutting instrument through the cured ink to the surface of the substrate. A device for this purpose is available from the Gardner/Neotec Instrument Division of Pacific Scientific. A pressure-sensitive tape, such as 3M Scoth®⁽¹⁰⁾ Brand 810, is applied over the grid pattern and then removed with a continuous, nonjerking motion. Depending on the extent of ink removal, the adhesion is rated from 0B to 5B, the highest rating representing no ink removal.

[0014] Many of the inks which fail this crosshatch test nevertheless exhibit acceptable adhesion in a simple tape-pull test. This implies that adhesion loss is due to delamination of the ink from the substrate due to the excess energy imparted to the ink during the cutting operation. Unless this energy can be stopped from traveling laterally across the ink-substrate interface, these inks will give poor crosshatch adhesion. It is frequently observed that inks with nominal crosshatch adhesion pass or fail depending on the type of cutting pattern; few cuts widely spaced impart less energy than several cuts close together on the same unit area. The ASTM test described above is designed to make crosshatch testing more reproducible by quantifying the transverse forces applied in any particular situation.

Prior Art

[0015] To survive the stress of crosshatching, polymeric inks need to be toughened so that the applied forces are absorbed or dissipated in the vicinity of the cuts and are thus prevented from traveling to the ink-substrate interface. One way of doing this is to increase the degree of crosslinking. Yet this technique can be counterproductive in that the resulting composition may become too brittle for a touch switch ink. Another method is to rubber-toughen the formulation with elastomeric inclusions, a technique widely used in epoxy chemistry. Yet a third method is to use rigid filler particles, such as alumina, silica and glass spheres. Recent studies reported have combined these last two approaches in epoxy systems to give hybrid-particulate composites. In these systems dispersed rubbery particles enhance the extent of localized plastic shear deformations around the crack tip, while the rigid particles increase crack resistance by a crack-pinning mechanism.

[0016] Preliminary work with an experimental rubber-toughned UV curable dielectric composition showed it to have excellent crosshatch adhesion to a rough surface such as Mylar®EL 500 polyester film, but not to a smoother surface such as Melinex®O polyester film. This can be explained by the greater surface area encountered by the dielectric in the former case, thus requiring additional force for delamination. Analogous compositions containing rigid filler particles but without the rubber fillers gave poor crosshatch adhesion to both rough and smooth polyester surfaces. A dielectric formulated as a hybrid-particulate composite, on the other hand, has been found to have excellent crosshatch adhesion to a wide variety of substrates, including a gamut of plasitc films with widely differing surfaces. The improved adhesion is attributed both to a rubber-­toughening mechanism and to crack-pinning of the filler. The rigid filler particles were found to contribute much less to the overall toughness (and, thus, adhesion) since the analogous compositions without the elastomeric inclusions give very poor crosshatch adhesion.

Detailed Description of the Invention

[0017] The invention is therefore directed to a novel UV-curable dielectric composition having excellent adhesion to a wide variety of polyester surfaces which contains both elastomeric and rigid fillers.

[0018] Such elastomeric fillers might be added to dielectric compositions in a variety of ways; for example, micron size core-shell polymers such as those disclosed by Burk in U.S. Patent 3,313,748 were blended with the dielectric. Another approach was to blend elastomeric polymers such as polyisoprene in the formulation. While both of these technical approaches were effective to some extent by far the most effective technique has been that of choosing monomers and oligomers which contribute both elastomeric and nonelastomeric character to the final composition.

[0019] Therefore, in accordance with the invention rubber fillers are incorporated into the composition by means of both an acrylated rubber modified epoxy resin oligomer and acrylated polybutadiene oligomer. The rheology of the system is then adjusted by the use of alkyl acrylates. A mixture of mono- and di-functional alkyl acrylates is particularly preferred for this purpose. Interestingly, even though the prior art would indicate that a wide variety of filler materials would function effectively, it has been found that the composition of the filler which can be used in the invention is quite critical. Only talc and mica have been found to be effective to attain the high degree of adhesion afforded by the composition of the invention.

A. Inorganic Adhesion Agent

[0020] A wide variety of inorganic fillers has been tested for use as an adhesion agent for the composition of the invention. (See Examples 3-14 infra.) However, the only materials which have been found to be generally effective are talc and mica.

[0021] The purity of the talc and mica does not seem to be critical and ordinary commercial grades of these materials are satisfactory for use in the invention. Unlike talc which has a single theoretical chemical composition (3MgO.4SiO₂.H₂O), mica occurs as several different forms of aluminum silicate of which muscovite and phlogopite have appreciable commercial usage. Either of these is suitable for use in he invention. Mixtures of talc and mica can be used without disadvantage.

[0022] At least 25% wt. of the talc and/or mica are required to obtain the deisred level of adhesion for the compositions of the invention. However, more than about 35% wt. of these adhesive agents is detrimental in that the cured composition may become too inflexible.

[0023] The talc and mica used in accordance with the invention may be treated with a silane coupling agent to effect bonding of the filler to the organic polymer constituents of the liquid curable component. This mainly improves the aging qualities of the composition, especially under environmental stress conditions.

[0024] Typical silane coupling agents have the structure R-Si

ORʹ)₃ in which R is an organo functional group which reacts with the organic polymer and ORʹ is an hydrolyzable group which hydrolyzes to yield (R-Si

OH)₃ which condenses with -Si-OH groups on substrates to yield an -Si-O-Si-bond. The various silanes contain different kinds of organofunctional groups. Available silane coupling agents include amino-functional silane, methacrylate-functional cationic silane, polyamino-­functional silane, mercapto-functional silane, vinyl-­functional silane and chloroalkyl-functional silane.

B. Epoxy Resin Oligomer

[0025] An essential ingredient of the curable liquid component of the invention and the primary rubbery material is the acrylated rubber-modified epoxy resin oligomer. These materials are prepared by reacting the epoxide moieties of a polyepoxide with the acid moieties of an unsaturated monocarboxylic acid and a liquid carboxyl-terminated homopolymer or copolymer of a conjugated diene. The preparation of these materials is described in US 3,892,819 to Najvar and in U.S. 3,928,491 to Waters. A preferred oligomer of this type is the reaction product of a bisphenol A-derived epoxy resin with acrylic acid and a carboxyl-terminated butadiene/acrylonitrile copolymer. The acrylated rubber-modified epoxy resin oligomer should constitute 20-50% wt. of the curable liquid component and preferably 35-45% wt.

C. Acrylated Polydiene Oligomer

[0026] A second essential ingredient of the curable liquid component and the secondary rubbery material is the acrylated polydiene oligomer. These materials are acrylates, normally diacrylates, of low molecular weight liquid conjugated diene/oligomers having a moleuclar weight of 2-4,000. A molecular weight of 3,000 has been particularly effective. Vinyl content of the oligomers is on the order of 15-30% wt., 20-25% wt. vinyl content being preferred. Acrylated oligomers of either butadiene or isoprene can be used in this application.

[0027] The polydiene oligomer should be 5-25% wt. of the composition and is preferably used in a lesser amount than the epoxy resin oligomer. From 7 to 20% wt. of the acrylated polydiene, particularly polybuta­diene, is especially preferred.

D. Alkyl Acrylates

[0028] Alkyl acrylates in some instances constitute a major part of the curable liquid component of the invention. In all case, the alkyl acrylates must be liquid at room temperature. Both mono- and multi-­functional acrylates can be used in the invention. However, the amount of tri- and higher functionality acrylates must be limited to 10% wt. or less of the curable liquid component in order to avoid excessive crosslinking and shrinkage of the composition. It is therefore preferred to employ only mono- and di-functional liquid alkyl acrylates in an amount of 35-80% wt. of the total curable liquid component. From 40 to 60% wt. is still further preferred.

[0029] Quite surprisingly, better adhesion results have been obtained using a mixture of mono-functional acrylates (30-60%) and di-functional acrylates (5-20%). More nearly optimum properties have been obtained when the mono-functional and di-functional acrylates constitute respectively 35-45% wt. and 7.5-15% wt. of the curable liquid component.

[0030] Suitable alkyl acrylates include but are not limited to acrylates and the corresponding methacrylates listed below:
allyl acrylate
tetrahydrofurfuryl acrylate
triethyleneglycol diacrylate
ethyleneglycol diacrylate
polyethyleneglycol diacrylate
1,3-butyleneglycol diacrylate
1,4-butanediol diacrylate
diethyleneglycol diacrylate
1,6-hexanediol diacrylate
neopentylglycol diacrylate
2-(2-ethoxyethoxy)ethyl acrylate
tetraethyleneglycol diacrylate
pentaerythritol tetraacrylate
2-phenoxyethyl acrylate
ethoxylated bisphenol A diacrylate
trimethylolpropane triacrylate
glycidyl acrylate
isodecyl acrylate
dipentaerythritol monohydroxypenta acrylate
pentaerythritol triacrylate
2-(N,N-diethylamino)ethyl acrylate
hydroxy lower alkyl acrylates such as, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyhexyl acrylate
benzoyloxyalkyl acrylates such as benzoyloxyethyl acrylate and
benzoyloxyhexyl acrylate
cyclohexyl acrylate
n-hexyl acrylate
dicyclopentenylacrylate
N-vinyl-2-pyrrolidone
isobornyl acrylate
isooctyl acrylate
n-lauryl acrylate
2-butoxyethyl acrylate
2-ethylhexyl acrylate
2,2-methyl-(1,3-dioxolan-4-yl)methyl acrylate.

[0031] In the case of momofunctional acrylates, it is preferred that they be of higher molecular weight and therefore of lower volatility. As can be seen from the above list, the alkyl moiety of the acrylate can be substituted with virtually any inert, organic group so long as the resultant acrylate remains liquid at room temperature and is miscible in the above-described acrylated polydiene oligomers. A preferred alkyl acrylate combination is dicyclo­pentenyloxyethyl acrylate and tripropyleneglycol diacrylate. (See Examples 1 and 2).

E. Additives

[0032] In addition to the above-described primary constituents, the composition of the invention may also contain various secondary materials to add to or enhance its properties such as elastomeric polymers, initiators to render the compositon UV curable, pigments (soluble or insoluble) and various printing aids such as leveling agents, anti-foam agents and thickeners. These materials are well known in the art and do not constitute a criterion on which the nonobviousness of the invention is based.

F. Formulation

[0033] The compositions of the invention are not difficult to formulate in that simple low energy mixing is sufficient to facilitate solution. While it is necessary that the compositions form stable admixtures, it is not necessary that the compositions be completely soluble in each other. In fact, some immiscibility of these blends was anticipated, which, upon UV curing, would lead to microscopic phase separation and, hence, to a multiple phase structure.

G. Test Procedures

[0034] Polyester film substrates employed for adhesion testing are commercially available 5 mil thick (127 µm) films. The several grades evaluated are specified in the examples. The polyimide substrate is also commercially available 5 mil thick (127 µm) film sold under the tradename Kapton®⁽³⁾ by the Du Pont Company. The polycarbonate film is commercially available 5 mil thick (127 µm) from General Electric under the tradename Lexan®⁽⁴⁾. The polymeric silver conductive ink is commercially available as product 5007⁽⁹⁾ from the Du Pont Company.

[0035] Prints measuring 1 square inch were made through a 280 mesh stainless steel screen to give 1 to 1.1 mil (25.4 to 27.9 µm) thick test patterns. Adhesion tests to silver were made over 5007 silver conductor previously cured over Mylar®EL 500 polyester film. The 5007 was printed with a 280 mesh stainless steel screen and cured at 120°C for 10 minutes. Silver print thickness was 0.5 to 0.7 mils (12.7 to 17.8 µm).

[0036] Adhesion results reported refer to crosshatch adheson run according to ASTM D3359-78, using Method B, in which a lattice pattern of 11 cuts in each direction is made in the dielectric to the substrate, pressure-sensitive tape is applied over the lattice and then removed, and the adhesion rated according to the degree of removal, according to he following scale:

5B The edges of the cuts are completely smooth; none of the squares of the lattice is detached.

4B Small flakes of the coating are detached at intersections; less than 5% of the area is affected.

3B Small flakes of the coating are detached along edges and at intersections of cuts. The area affected is 5 to 15% of the lattice.

2B The coating has flaked along the edges and on parts of the squares. The area affected is 15 to 35% of the lattice.

1B The coating has flaked along the edges of cuts in large ribbons and whole squares have detached. The area affected is 35 to 65% of the lattice.

0B Flaking and detachment worse than Grade 1.

All adhesion tests were run with 3/4 inch wide 3M Scothc® tape #810, using a Cross Hatch Cutter from the Gardner/Neotec Instrument Division of Pacific Scientific with a medium blade (eleven teeth with 1.5 mm spacings).

[0037] All dielectric prints were cured under ultraviolet light on an RPC Industries QC®⁽⁸⁾ Processor Model 1202 AN, containing two 200 W/linear inch (79 W/linear cm) medium pressure mercury vapor light bulbs, running at 40 ft/min (20.3 cm/sec.); the samples were cured in air approximately 3 inches from the lamps.

EXAMPLES

[0038] Two initial compositions in accordance with the invention were formulated using talc and mica respectively as the rigid filler adhesive agents (Examples 1 and 2). Then a series of twelve more compositions was made in which other well-known rigid fillers were substituted for the mica and talc. A list of the rigid fillers used in the 20 examples is given in Table I below while the adhesive properties of each formulation are given in Table II.

[0039] In Examples 15-20 several adhesive compositions were formulated to show various criticalities with respect to the liquid component. Finally in Example 21 a composition was formulated identical to Example 1 except that the rigid filler was omitted altogether.

[0040] Additional data for all 20 examples in which a wide variety of substrates was tested are given in Table II.

Example 1

[0041] A UV curable mixture was made from 26.09% wt. of an acrylated rubber-modified epoxy resin, 7.34% wt. of an acrylated polybutadiene oligomer, 26.22% wt. of dicyclopentenyloxyethyl acrylate, 6.52% wt. of tripropyleneglycol diacrylate, 0.17% of a predispersed copper phthalocyanine pigment in trimethylolpropane triacrylate (20:80), 2.44% wt. of 2-hydroxy-2-methyl-1-phenyl-1-propanone, 0.69% wt. of 2,2-diethoxyacetophenone, 0.53% wt. of a silicone printing aid, and 30.0% wt. talc. After printing and curing, this composition gave excellent crosshatch adhesion over a broad spectrum of substrates, as shown in Table II.

Example 2

[0042] Example 1 is repeated, except that mica is used in place of talc. After printing and curing, this composition also gave excellent crosshatch adhesion over a broad spectrum of substrates, as shown in Table II.

Examples 3-14

[0043] Example 1 is repeated, except that talc is replaced by other filler candidates, as shown in Table II. These compositions do not show the excellent adhesion to a wide spectrum of substrates shown in Examples 1 and 2, in which talc and mica were used.

Example 15

[0044] Example 1 is repeated, except that the acrylated rubber-modified epoxy resin is replaced by an acrylated epoxy resin. This composition does not show the excellent adhesion to a wide spectrum of substrates shown by Example 1.

Example 16

[0045] Example 1 is repeated, except that the acrylated rubber-modified epoxy resin is replaced by an acrylated aromatic urethane resin. This composition does not shown the excellent adhesion to a wide spectrum of substrates shown by Example 1.

Example 17

[0046] Example 1 is repeated, except that the acrylated polybutadiene oligomer is replaced by an equivalent amount of tripropylene glycol diacrylate. This composition does not show the excellent adhesion to a wide spectrum of substrates shown by Example 1.

Example 18

[0047] Example 1 is repeated, except that the acrylated polybutadiene oligomer and the dicyclopentenyloxyethyl acrylate are both replaced by an equivalent amount of tripropylene glycol diacrylate. This compostion does not show the excellent adhesion to a wide spectrum of substrates shown by Example 1.

Example 19

[0048] Example 1 is repeated, except that the dicyclopentenyloxyethyl acrylate is replaced by an equivalent amount of tripropylene glyco diacrylate. This composition does not show the excellent adhesion to a wide spectrum of substrates shown by Example 1.

Example 20

[0049] Example 1 is repeated, except that the acrylated polybutadiene oligomer and the tripropylene glycol diacrylate are both replaced by dicyclopentenyloxyethyl acrylate. This composition does not show the excellent adhesion to a wide variety of substrates shown by Example 1.

Example 21

[0050] Example 1 is repeated except that the talc was omitted from the composition. This composition did not shown adequate adhesion to the wide variety of substrates as did the corresonding talc containing compositions of Examples 1 and 2.

Example 22

[0051] A UV curable mixture was made from 13.30% wt. of an acryalted rubber-modified epoxy resin, 13.23% wt. of an acrylated polybutadiene oligomer, 36.33% wt. of dicyclopentenyloxyethyl acrylate, 3.31% wt. of tripropyleneglycol diacrylate, 0.17% of a predispersed copper phthalocyanine pigment in trimethylolpropane triacrylate (20:80), 2.44% wt. of 2-hydroxy-2-methyl-1-phenyl-1-propanone, 0.69% wt. of 2,2-diethoxyacetophenone, 0.53% wt. of a silicone printing aid, and 30.0% wt. talc. After printing and curing, this compostion gave excellent crosshatch adhesion over a broad spectrum of substrates, as shown in Table II.

Example 23

[0052] A UV curable mixture was made from 38.94% wt. of an acrylated rubber-modified epoxy resin, 3.31% wt. of an acrylated polybutadiene oligomer, 14.23% wt. of dicyclopentenyloxyethyl acrylate, 9.73% wt. of tripropyleneglycol diacrylate, 0.17% of a predispersed copper phthalocyanine pigment in trimethylolpropane triacrylate (20:80), 2.44% wt. of 2-hydroxy-2-methyl-1-phenyl-1-propanone, 0.69% wt. of 2,2-diethoxyacetophenone, 0.53% wt. of a silicone printing aid, and 30.0% wt. talc. After printing and curing, this composition did not give the excellent crosshatch adhesion over a broad spectrum of substrates, as shown in Table II.

Example 24

[0053] A UV curable mixture was made from 38.94% wt. of an acrylated rubber-modified epoxy resin, 3.31% wt. of an acrylated polybutadiene oligomer, 10.72% wt. of dicyclopentenyloxyethyl acrylate, 13.23% wt. of tripropyleneglycol diacrylate, 0.17% of a predispersed copper phthalocyanine pigment in trimethylolpropane triacrylate (20:80), 2.44% wt. of 2-hydroxy-2-methyl-1-phenyl-1-propanone, 0.69% wt. of 2,2-diethoxyacetophenone, 0.53% wt. of a silicone printing aid, and 30.0% wt. talc. After printing and curing, this composition also did not give excellent crosshatch adhesion over a broad spectrum of substrates, as shown in Table II.

[0054] The composition of Example 1, which is the best mode of the invention, has quite excellent performance properties. These are shown in Table III which follows.

Tradenames

[0055] 

(1) Chemlink® 5000 is a tradename of Sartomer Company, West Chester, PA for acrylated butadiene liquid oligomer

(2) Hycar® is a tradename of B. F. Goodrich Chemicals, Inc., Akron, OH for carboxyl-terminted liquid polymers

(3) Kapton® is a tradename of E. I. du Pont de Nemours and Company, Wilmington, DE for polyimide films

(4) Lexan® is a tradename of General Electric Co., Schenectady, NY for polycarbonate film

(5) Luminar® is a tradename of Toray Industries, Inc., Tokyo, Japan for polyester film

(6) Melinex® is a tradename of ICI Americas, Inc. for polyester film

(7) Mylar® is a tradename of E. I. du Pont de Nemours and Company, Wilmington, DE for polyester films

(8) QC is a tradename of RPC Industries, Inc., Plainfield, IL for UV light curing apparatus

(9) 5007 is a designation of E. I. du Pont de Nemours and Company, Wilmington DE for polymeric silver conductive ink

(10) Scotch® is a tradename of 3M Corporation, Minneapolis, MN for pressure-sensitive adhesive tape.

Claims

1. A printable dielectric composition comprising

(a) 25-35% wt. finely divided particles of an inorganic adhesion agent selected from talc, mica and mixtures thereof dispersed in

(b) 75-65% wt. curable liquid composition comprising:
(1) 20-50% wt. acrylated rubber-modified epoxy resin oligomer;
(2) 5-25% wt. acrylated polydiene oligomer; and
(3) 35-75% wt. alkyl acrylate.

2. A printable dielectric composition comprising

(a) 25-35% wt. finely divided particles of an inorganic adhesion agent selected from talc, mica and mixtures thereof dispersed in

(b) 75-65% wt. curable liquid composition comprising:
(1) 20-50% wt. acrylated rubber-modified epoxy resin oligomer;
(2) 5-25% wt. acrylated polybutadiene oligomer;
(3) 30-60% wt. monofunctional alkyl acrylate; and
(4) 5-15% wt. difunctional alkyl acrylate.

3. The composition of claim 2 in which the proportions of the components in the curable liquid composition are:

(1) 35-45% wt.

(2) 7.5-15% wt.

(3) 35-45% wt.

(4) 7.5-15% wt.

4. The composition of claim 1 which contains up to 5% wt. inert pigment.

5. The composition of claim 1 which is UV curable and contains 0.1-10% wt. photoinitiator.

6. The composition of claim 1 in which the inorganic particles are treated with a silane coupling agent.

7. The composition of claim 1 which contains additionally 0.1-2.0% by weight of a printing aid.

8. A membrane touch switch comprising upper and lower flexible layers having facing electrically conductive areas separated by an adherent spacer layer of the composition of claim 1.

9. A membrane touch swith comprising upper and lower flexible layers having facing electrically conductive areas separated by an adherent spacer layer and having electrically conductive traces leading therefrom encapsulated within a layer of the composition of claim 1.

10. A membrane touch switch comprising upper and lower flexible dielectric polymer layers at least one of which layers has a plurality of overlying electrically conductive areas each separated from the other by a layer of the composition of claim 1.